1. Field of the Invention
The present invention relates to an optical storage apparatus for storing information, in the form of a large number of pits, on the surface of an optical disk, the pits being aligned in tracks. The present invention further relates to a servo system for tracking an optical beam, such as a laser beam, on the surface of an optical disk to read out information stored on the disk or to write information on the disk.
2. Description of the Related Art
Optical storage devices usually store binary signals in the form of a large number of pits aligned in circular tracks on an optical disk. The tracks may be pregrooves (not shown) which are raised from the surface of the disk. The information is optically read out by scanning the disk with a laser beam having a diameter on the order of 1 .mu.m. The stored information is read by sensing (or detecting) the laser beam after it is reflected and modulated by the surface of the disk. The beam is modulated by optical defraction due to the presence of the pits formed on the optical disk.
The use of an optical storage medium, i.e., an optical disk, is desirable because it is possible to achieve a much higher signal density with an optical storage medium than with a magnetic storage medium; a high signal density being achieved because of the fine and precise structure of the optical medium which forms the optical disk. Further, the pitch of the tracks is substantially small, typically 1.6 .mu.m, and the size of the pits aligned on the tracks is on the order of 0.1 .mu.m. Thus, to accurately read information from an optical disk, a laser beam must be precisely focused and centered on a track. Consequently, an accurate servo system is required to focus and center the laser beam on a track.
The tracks on an optical disk may be eccentric, mainly due to insufficient dimensional preciseness of the disk, and have positional variations of up to 100 .mu.m. Thus, the tracks on a rotating disk have periodic and wide ranging positions in the radial direction, which creates substantial difficulty in performing accurate tracking. In addition, an objective lens with a high numerical aperture, such as 0.5, is utilized to detect, or perceive, extremely fine details, leading to a very small depth of field, on the order of several .mu.m. Further, the disk surface may have variations in the vertical direction on the order of 100 .mu.m due to the distortion of the disk, leading to difficulty in focusing.
In spite of the above-mentioned adverse conditions, the laser beam must be tracked with an accuracy of approximately 0.1 .mu.m in order to avoid cross-talk, and the laser beam must be focused on the surface with an accuracy of approximately 0.1 .mu.m. Thus, an accurate servo system for centering a laser beam on the track is the key to an optical storage apparatus. Various servo systems for focusing and tracking an optical beam have been developed and are reported in references such as "Optical Readout of Video Disks," by C. Bricot et al., IEEE Transaction C.E., November 1976, p. 304.
Tracking servo systems also experience problems due to "beam shift" which is inherent in most conventional optical systems employed in optical storage devices. "Beam shift" is enhanced by a mechanical hazard referred to as "skew" of the disk surface, caused by slight radial distortions, usually convex, of optical disks. Optical servo systems are designed so that the incident laser beam is perpendicular to the disk surface; accordingly, the reflected laser beam follows the same path as the incident laser beam. If the incident laser beam is not perpendicular to the surface of the disk, the return path of the reflected laser beam will be different from the path of the incident laser beam, resulting in beam shift which adversely affects the tracking servo system.
Conventional servo systems are described below, with reference to FIGS. 1-3, in order to explain beam shift.
FIG. 1 is a schematic block diagram of one example of a conventional system for obtaining tracking error signals. This system includes an objective lens 2 and an actuator 3 for driving the objective lens 2. The actuator 3 includes electrodynamic coils, similar to those used in audio speakers, to drive the lens 2 in a tracking direction T in accordance with tracking error signals, and in a focus direction F in accordance with focus error signals. The actuator 3 is referred to as a two-dimensional actuator. A laser beam 6 following a beam path perpendicular to the disk 1 passes through a beam splitter 4 and is focused on the disk 1 by lens 2 to form a focal point 7. The laser beam is then reflected and follows the path of the incident laser beam. Subsequently, a portion, or part, of the laser beam is deflected at a right angle by the beam splitter 4 and follows beam path 6-1 to a photosensor 5 comprising two sub-photosensors A and B. The aperture of the objective lens 2 forms a spot 15 at a fixed position on the photosensor 5. When the laser beam 6 is not well centered on the track, the intensity distribution of the laser beam over the spot 15 is asymmetrical due to the optical defraction of the deflected laser beam. The asymmetry of the beam intensity is sensed by the photosensor 5 and converted into tracking error signals by a differential amplifier (not shown) connected to sub-photosensors A and B. This method of generating tracking error signals is referred to as a "push-pull" method.
The occurrence of beam shift will now be described. When lens 2 is moved to a different position, as shown by the dotted lines and reference numeral 2' in FIG. 1, by the actuator 3 in accordance with a tracking error signal, the focal point 7 of the lens is moved to a new position 7'. Thus, the path of the incident laser beam is not perpendicular with the surface of the disk 1. Consequently, the reflected laser beam follows a dotted line path 6'-1 and is shifted by a distance s from the original path 6-1. This is the phenomenon of beam shift. As a result, the optical axis of the laser beam incident on the photosensor 5 is shifted by the distance s, and the intensity distribution of the laser beam over the spot 15 is changed, generating an off-set (a fixed deviation of the value of the signal current) in the tracking error signals. Therefore, it is impossible to perform an exact servo tracking operation.
Another example of beam shift will be described with reference to FIG. 2. The tracking servo system of FIG. 2 has two actuators, a focus servo actuator and a tracking servo actuator 8. An objective lens 2 is driven only in a focusing direction F by the servo focusing actuator 3' to focus the laser beam 6, which follows a path 6-2, on the disk 1. The laser beam 6 is tracked by pivoting mirror 9, and a servo tracking actuator 8, driven by a tracking error signal, controllably rotates the mirror 9 around a pivot axis P'. When the center of a track is in line with the optical axis of lens 2, a laser beam 6 following the solid line path 6-1 is perpendicular to the disk 1 and the reflected laser beam will return along path 6-2. However, when the eccentricity of the disk 1 causes the track to move to a different position 7', the mirror 9 must be rotated to a new position 9', shown by the dotted lines in FIG. 2, and the laser beam 6 follows optical path 6'-2, shown by the dotted line. In this case the laser beam incident on the disk 1 is not perpendicular to the surface of the disk 1, and thus a beam shift of a distance s results in an erroneous tracking error signal.
In order to alleviate beam shift, an improved structure for a tilting mirror has been proposed by Maeda et al. in the conference bulletin of the Japanese Applied Physics Society, 7P-X-8, April 1983. The blook diagram of FIG. 3 illustrates this structure, in which the pivot axis 12 of a tilting mirror 13, which is driven by a mirror actuator 14, is located on a back focal plane 11 of the objective lens 2. The back focal plane 11 is a plane which is parallel to the surface of the disk 1 and which includes a back focal point 10 of lens 2, the back focal point of lens 2 being on the opposite side of lens 2 from disk 1. A laser beam 6, following beam path 6-3, is deflected by tilting mirror 13 to pass in the immediate vicinity of the back focal point 10 of lens 2. As a result, the beam shift is negligibly small. The lens 2, however, is moved in a focusing direction F by a focusing actuator 3. This movement of the lens 2 results in a small discrepancy between the pivot axis 12 of the tilting mirror 13 and the back focal plane 11 of lens 2. The beam shift due to the discrepancy of the back focal plane 11 and the pivot point 12 has been shown to be negligibly small. Thus, the improved structure of the tilting mirror 13 provides a solution to the problem of beam shift in a tracking servo system. However, the tilting mirror 13, including its associated actuator 14, is rather large, adversely affecting the ability of the servo apparatus to respond to a tracking error signal of high frequency. The size of the tiling mirror 13 also adversely affects the density of the components of the servo apparatus.
In addition to the aforesaid problems inherent to optical tracking servo systems, the skew of the optical disk 1 is a problem which is difficult to overcome. Accordingly, it is important to alleviate the problem of beam shift.